With the recent advances in the techniques of performing digital encoding of various types of information such as picture information and the broadband network techniques, applications using these techniques have been increasingly developed, and systems for transmitting compress-encoded pictures and the like by using communication networks have been developed.
For example, a videophone, a teleconference system, and a digital television broadcast use a technique of compress-encoding video information and speech information to small information amounts, multiplexing the compressed video code stream, the compressed voice code stream, and other data code streams into one code stream, and transmitting/storing it.
As video signal compress-encoding techniques, motion compensation, discrete cosine transform (DCT), sub-band encoding, pyramid encoding, variable word-length encoding, and the like and schemes combining these techniques have been developed. The national standard schemes for video encoding include ISO MPEG1, MPEG2, ITU-T H.261, H.262, and H.263. The national standard schemes for multiplexing code streams obtained by compressing video signals and speech/audio signals and other data include the ISO MPEG system, ITU-TH.221 and H.223.
In a conventional video encoding scheme such as one of the above video encoding national standard schemes, a video signal is segmented into frames, and each frame is further segmented into small regions, thus encoding the signal in units of GOBs, macroblocks, or the like. Header information indicating the encoding mode and the like is added to each frame, GOB, and macroblock. These pieces of header information are always required to decode the overall GOBs and the like. If, therefore, a transmission path/storage medium error is introduced into header information to cause a decoding failure in a video decoder, the overall frame, GOB, or the like to which the header information is added cannot be properly decoded. As a result, the quality of the reconstructed picture in the video decoder greatly deteriorates.
When compress-encoded picture data is to be transmitted by using a communication network, the reception side must decode the data to reconstruct significant information from the transmitted bit stream of “0”s and “1”s. For this purpose, the above header information plays a very important role as information indicating a specific rule under which a predetermined block of bit streams is encoded. Such header information contains, for example, information indicating the prediction type of the currently encoded frame (intraframe encoding, interframe encoding, or the like), information indicating the display timing (time reference) of the frame, step size information required for quantization, and the like. If these pieces of header information are lost, information transmitted afterward cannot be properly decoded.
Assume that an error is introduced into a bit stream owing to some cause to change information indicating the prediction type of a frame from interframe encoding to intraframe encoding. In this case, even if actual information is properly transmitted afterward, since it is determined on the decoding side that the signal is obtained as a result of intraframe encoding, the signal is not properly decoded in the end. The quality of the reconstructed picture in the video decoder therefore greatly deteriorates.
Systems using wire communication networks have been prevalent to date. Even systems using radio communication systems have been built on the assumption that they use satellite communication with very low error rates. Sufficient consideration has not therefore been given to error robustness of the structure of a code stream to be transmitted, and satisfactory protection has not been given against transmission path errors in important information such as header information.
In a PHS system which is part of the mainstream of mobile communication, the error rate is about hundred thousand or one million times that in satellite communication. For this reason, sufficient correction cannot be performed by only error correction with respect to bit streams encoded by a conventional technique. In an Internet system which is expected to become part of the mainstream of communication like a PHS system, the types of errors that are likely to occur in data have not been statistically clarified, and hence proper error correction cannot be performed in some case. Furthermore, in the PHS and Internet systems, part of information in a code stream may be lost. Theoretically, error correction cannot cope with such a case. For this reason, the structure of a code stream itself needs to have error robustness.
As described above, satisfactory consideration has not been given to error robustness of the structure of a code stream to be transmitted. Satisfactory consideration about transmission path errors has not been given to important information such as header information, in particular, whose picture quality greatly deteriorates when a transmission path error is introduced therein.